Complementation property:
DESk(m) = (DESk'(m'))'
where x' is the bitwise complement of x;

Differential cryptanalysis: first version unusable as at 16 rounds, more
plaintext/ciphertext pairs needed than exhaustive key trial; but for 15
rounds, cuts this time. Later versions cut it to 247
tries. Works by comparing xors of results with xors of corresponding
plaintext. Designers of DES knew about this one, hence the design of
the S-boxes

Linear cryptanalysis drops required chosen plaintext/ciphertext pairs to
242; not known to designers of DES.